Flip an image across diagonal. Uses perspective transform.

diagonal_flip.py

#!/usr/bin/env python
from gimpfu import *


def diagonal_flip(image, drawable, downward_diag, upward_diag):
    pdb.gimp_image_undo_group_start(image)
    if downward_diag:
        pdb.gimp_item_transform_perspective(drawable, 0, 0, 0, drawable.height,
            drawable.width, 0, drawable.width, drawable.height)

    if upward_diag:
        pdb.gimp_item_transform_perspective(drawable, drawable.width, drawable.height,
            drawable.width, 0, 0, drawable.height, 0, 0)

    pdb.gimp_image_undo_group_end(image)


register(
    'diagonal_flip',
    'Flip layer along diagonal',
    'Flip layer along diagonal',
    'Michael Shoemate',
    'Michael Shoemate',
    '2017',
    "Diagonal Flip",
    "*",
    [
        (PF_IMAGE, "image", "Input image", None),
        (PF_DRAWABLE, "drawable", "Input drawable", None),
        (PF_BOOL, "downard_diag", "Downward Diagonal", 0),
        (PF_BOOL, "upward_diag", "Upward Diagonal", 0)
    ],
    [],
    diagonal_flip,
    menu="<Image>/Filters/Custom")

main()

Only works for square images. I could make a new layer with switched dimensions, but I don't actually need it...